1. Field of the Invention
This invention relates to an ophthalmic measurement apparatus, and more particularly to an ophthalmic measurement apparatus which irradiates the interior of a patient's eye with a beam of laser light and uses the scattering of laser beam from the interior of the eye to output measurement quantities such as protein concentration in the anterior chamber of the eye.
2. Description of the Prior Art
Measurement of protein concentration in the anterior chamber of the eye is of considerable importance in determining whether the camera oculi is inflamed, that is, whether the blood-aqueous barrier function is normal or not. In one method that is frequently used for this a slit lamp microscope is employed to grade the concentration by observation with the naked eye, while in another method photographic techniques are used to obtain quantitative measurements. However, as yet there is no method that is easy to use clinically.
Data obtained with the conventional method of naked-eye measurement lacks reliability as judgments vary depending on the person making the measurement. One solution has been to use a method in which a beam of laser light is projected into the eye and the light scattering from the eye is detected and subjected to quantitative analysis.
Examples of such an ophthalmic measurement apparatus which irradiates the eye with a beam of laser light and detects the light scattered from the eye are disclosed in Japanese Patent Laying-open Nos. 120834/87 (corresponding to U.S. patent application Ser. No. 926,650 filed on Mar. 11, 1986) and 135128/88 (corresponding to U.S. Pat. No. 4,832,043). In such an arrangement, the beam from a laser light source is focused on a prescribed point in the eye such as in the anterior chamber, for example, and scattered light from the eye is detected, via a mask with a rectangular aperture of a prescribed size, by a photosensor which converts the light to an electrical signal which is processed to determine protein concentration or other such ophthalmic measurement quantities.
The extremely low intensity of the scattered laser light makes it susceptible to noise in the form of light other than light from the region of interest. Taking the detection of the anterior chamber as an example, if the measurement area is too close to the crystalline lens, light scattering from the crystalline lens will be picked up as noise which will affect the results.
To reduce or eliminate the effects of such noise, in the apparatus described in Japanese Patent Laid-open No. 135128/88 the laser beam is made to overscan the mask aperture and the difference is obtained between a signal obtained from the photosensor when the laser beam is within the limits of the aperture and a signal obtained when the beam is outside the aperture.
The human cornea has a strong lens effect which causes incident light that is not along the normal line to be refracted at the cornea surface. Hence, when the area on which the light impinges is changed the degree of refraction also changes, disturbing the relationship between the measurement area (point of laser beam convergence) and the mask of the light receiving section.
The depth of the aqueous humor in the anterior chamber is around 3 mm, thus the laser has to be focused on a median portion measuring 1 mm to 2 mm and the light scattered from this measurement area has to be accurately captured. This necessitates accurate alignment of the apparatus with the patient's eye, particularly in the horizontal plane, and a method of accurately achieving this alignment. Failure to align the system prior to carrying out measurements will result in the entry into the measurement mask aperture of various undesired light rays from areas other than the measurement area concerned, making it impossible to obtain accurate measurements.